Several methods exist to make porous membranes. Wet and dry phase inversion are among the most widely applied techniques for making membranes with small pore sizes in the range of 0.1 nm to 1 μm. In the wet phase inversion method a polymer is dissolved in a solvent and contacted with a non-solvent, usually by immersion, to initiate precipitation of the polymer: the solid polymer-rich phase forms the membrane structure while the liquid solvent rich phase forms the pores. In the dry phase inversion method the polymer is dissolved in a mixture of a solvent and a non-solvent whereby the solvent is more volatile than the non-solvent and by evaporation of the solvent the polymer starts to precipitate in the non-solvent. Both methods are limited to specific combinations of polymers, solvents and non-solvents and because of relatively long contact times especially the wet phase inversion process is rather slow. And also because organic solvents are used environmental and safety issues make manufacturing troublesome and expensive.
Alternatively by the technique of ultraviolet or electron beam irradiation porous membranes or porous materials can be produced as is described in e.g. U.S. Pat. No. 4,466,931, U.S. Pat. No. 6,126,189, EP-A-216 622, EP-A-481 517 and EP-A-1 533 321. In practice a variety of organic solvents are applied to dissolve the monomers.
The technique of making porous structures by free radical polymerization (thermally or by UV irradiation) is disclosed in GB-A-932 126.
Others use free radical polymerization by UV irradiation for polymerization of the monomers and make the membrane by the conventional technique of wet phase inversion, as described in e.g. EP-A-430 082 and EP-A-803 533.
Water soluble (also referred to as water reducible) epoxy acrylates are described for making non-porous coatings and films as disclosed in e.g. WO-A-02/081576, WO-A-2005/066231 and WO-A-2004/060689.
In the process of continuous optimization of membrane manufacturing there is a need for a fast, simple and efficient method that does not require extensive investments related to the field of environmental regulations and safety.
Several examples can be found in which the curing technique is used to produce inkjet recording media. WO-A-2001/092023, WO-A-03/016068 and WO-A-97/33758 disclose membranes comprising inorganic particles needed to achieve the porous character.
EP-A-0 704 315 mentions the possibility to improve the water resistance by adding a curable resin to the cationic resin layer of an inkjet recording medium. EP-A-0 547 806 describes an authentication-identification card wherein a writing layer contains a binder that can be a UV curable resin. However, in neither EP-A-0 704 315 nor EP-A-0 547 806 porosity is created by phase separation of the cured polymer.
In EP-A-1 401 664, WO-A-2006/037085 and US-A-2005/153147 inkjet recording media are disclosed in which one or more ink receptive layers are coated on top of a radiation-curable layer.
Another method to obtain porosity is the application of foamed layers as in for example EP-A-0 888 903.
In yet other methods to make inkjet recording media coated layers are irradiated by ultraviolet or electron beam radiation, but these layers are not porous as in e.g. EP-A-709 438, WO-A-99/42296, WO-A-99/21723, GB-A-2182046 and WO-A-2001/091999.
Where porous membranes are used for making recording media high concentrations of organic solvents are required as described in e.g. JP-A-2002178630 and JP-A-2001010221.
International application PCT/NL2006/000154 describes a process for making microporous membranes by curing compounds through radiation, as well as image recording materials, in which these microporous membranes are used.
One of the important properties of an ink receptive coating formulation is the liquid absorptivity. The majority, if not all, of the ink solvent has to be absorbed by the coating layer itself. Only when paper, cloth or cellulose is used as a support, some part of the solvent may be absorbed by the support. Another important property for an inkjet recording medium having photographic quality, is the optical density of the images printed thereon.
When comparing the known solutions for providing an inkjet recording medium, including media with a porous receiving layer and media with a water swellable layer, these solutions have their positive and negative characteristics.
On the one hand, the swellable type of inkjet recording media may exhibit high densities, but these generally dry slowly. On the other hand the porous inkjet recording media have excellent drying properties, but generally have lower densities than swellable media, especially those media that are based on polymeric porous layers.
There remains a strong need for ink-jet recording media having excellent drying properties and which show minimal dye fading and have high image densities. In addition, these ink-jet recording media should preferably have properties such as suitable durability, good sheet feeding property in ink-jet printers, good gloss, as well as a good resolution.
The present invention seeks to fulfill, at least in part, this need.